BGP Message Format

Marker

16 octets

Indicates whether the information synchronized between BGP peers is complete. This field is used for calculation in BGP authentication. If no authentication is used, the field is set to all ones in binary format or all Fs in hexadecimal notation.

Length

2 octets (unsigned integer)

Indicates the total length of a BGP message (including the header), in octets. The length ranges from 19 octets to 4096 octets.

Type

1 octet (unsigned integer)

Indicates the message type. The value of this field is an integer ranging from 1 to 5.

1

Open

2

Update

3

Notification

4

Keepalive

5

Route-refresh

Version

1 octet (unsigned integer)

Indicates the BGP version number. For BGP-4, the value of the field is 4.

My Autonomous System

2 octets (unsigned integer)

Indicates the AS number of the message sender.

Hold Time

2 octets (unsigned integer)

Indicates the hold time set by the message sender, in seconds. BGP peers use this field to negotiate the interval at which Keepalive or Update messages are sent so that the peers can maintain the connection between them. Upon receipt of an Open message, the finite state machine (FSM) of a BGP speaker must compare the locally configured hold time with that carried in the received Open message. The FSM uses the smaller value as the negotiation result. The value is greater than or equal to 3. A value of 0 indicates that no Keepalive messages are sent. The default value is 180.

BGP Identifier

4 octets (unsigned integer)

Indicates the router ID of the message sender.

Opt Parm Len

1 octet (unsigned integer)

Indicates the length of the Optional Parameters field. If the value is 0, no optional parameters are used.

Optional Parameters

Variable

Indicates a list of optional BGP parameters, with each one representing a unit in TLV format.

0               7              15
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
|  Parm. Type   | Parm. Length  |  Parameter Value (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...

• Parm. Type: indicates the parameter type. The value is an unsigned integer and occupies 1 octet. The field is valid only if its value is 2, which indicates that a capability needs to be negotiated.
• Parm. Length: indicates the length of Parameter Value. The value is an unsigned integer and occupies 1 octet.

• Parameter Value: varies with Parm. Type. If the value of Parm. Type is 2, Parameter Value indicates the list of capabilities that can be negotiated. Each unit in the list consists of the following TLV:

  +------------------------------+
  | Capability Code (1 octet)    |
  +------------------------------+
  | Capability Length (1 octet)  |
  +------------------------------+
  | Capability Value (variable)  |
  +------------------------------+

  • Capability Code: indicates a capability number and occupies 1 octet. If the value is 1, the address family capability is supported. If the value is 2, the route-refresh capability is supported.
  • Capability Length: indicates the length of Capability Value and occupies 1 octet.

  • Capability Value: varies with Capability Code.

    If the value of Capability Code is 1:

    Capability Value is a TLV and occupies 4 octets.

    0       7      15      23      31
    +-------+-------+-------+-------+
    |      AFI      | Res.  | SAFI  |
    +-------+-------+-------+-------+

    AFI: is short for address family identifier and occupies 2 octets. AFI is used with the subsequent AFI (SAFI) to determine the relationship between the network layer protocol and IP address. The encoding mode is the same as that in multiprotocol extensions. The value complies with the address family numbers defined in the related RFC protocol.

    Res: is reserved and occupies 1 octet. This field is ignored by the interface that receives the message. The value must be set to 0.

    SAFI: occupies 1 octet. SAFI is used with AFI to determine the relationship between the network layer protocol and IP address. The encoding mode is the same as that in multiprotocol extensions. The value complies with the address family numbers defined in the related RFC protocol.

    If the value of Capability Code is 2:

    The route-refresh capability is supported. The value of Capability Length is 0, and Capability Value is omitted.

    Devices can process Route-refresh messages only after the route-refresh capability is negotiated successfully. By default, the IPv4 unicast and route-refresh capabilities are supported.

1

IPv4 address family

1

Unicast

IPv4 unicast

2

Multicast

IPv4 multicast

128

VPN

IPv4 L3VPN

2

IPv6 address family

1

Unicast

IPv6 unicast

2

Multicast

IPv6 multicast

128

VPN

IPv6 L3VPN

196

Layer 2

128

VPN

BGP L2VPN

Unfeasible routes length

2 octets (unsigned integer)

Indicates the length of the Withdrawn Routes field, in octets. If the value is 0, the Withdrawn Routes field is omitted.

Withdrawn Routes

Variable

Contains a list of routes to be withdrawn. Each entry in the list contains the Length (1 octet) and Prefix (length-variable) fields.

• Length: indicates the mask length of the route to be withdrawn. The value 0 indicates a mask length that matches all routes.

• Prefix: contains an IP address prefix, followed by the minimum number of trailing bits needed to make the end of the field fall on an octet boundary. For example, consider the withdrawal of the route 192.168.200.200. The Prefix (in hexadecimal encoding) of the route varies according to different mask lengths:

  Mask (Decimal notation) Length (Hexadecimal notation) Prefix

  32 20 C4 A8 C8 C8

  25 19 C4 A8 C8 80

  20 14 C4 A8 C0

  15 0F C4 A8

Total Path Attribute Length

2 octets (unsigned integer)

Indicates the total length of the Path Attributes field. If the value is 0, both the Network Layer Reachability Information field and the Path Attributes field are omitted in the Update message.

Path Attributes

Variable

Indicates a list of path attributes in the Update message. The type codes of the path attributes are arranged in ascending order. Each path attribute is encoded as a TLV (<attribute type, attribute length, attribute value>) of variable length.

Figure 4 Format of the BGP path attribute TLV

Attr.TYPE occupies two octets (unsigned integer), including the one-octet Flags field (unsigned integer) and the one-octet Type Code field (unsigned integer).

Figure 5 TLV structure-Type

Attr.Flags: occupies one octet (eight bits) and indicates the attribute flag. The meaning of each bit is as follows:

O (Optional bit): defines whether the attribute is optional. The value 1 indicates an optional attribute, whereas the value 0 indicates a well-known attribute.

T (Transitive bit): Defines whether the attribute is transitive. For an optional attribute, the value 1 indicates that the attribute is transitive, whereas the value 0 indicates that the attribute is non-transitive. For a well-known attribute, the value must be set to 1.

P (Partial bit): Defines whether the information in an optional-transitive attribute is partial. If the information is partial, P must be set to 1; if the information is complete, P must be set to 0. For well-known attributes and for optional non-transitive attributes, P must be set to 0.

E (Extended Length bit): defines whether the length (Attr. Length) of the attribute needs to be extended. If the attribute length does not need to be extended, E must be set to 0 and the attribute length is 1 octet. If the attribute length needs to be extended, E must be set to 1 and the attribute length is 2 octets.

U (Unused bits): Indicates that the lower-order four bits of Attr. Flags are not used. These bits are ignored on receipt and must be set to 0.

Attr.Type Code: Indicates the attribute type code and occupies 1 octet (unsigned integer). For details about the type codes, see Table 6.

Attr.Value: Varies with Attr.Type Code.

Network Layer Reachability Information (NLRI)

Variable

Indicates a list of IP address prefixes in the Update message. Each address prefix in the list is encoded as a 2-tuple LV (<prefix length, the prefix of the reachable route>). The encoding mode is the same as that used for Withdrawn Routes.

1: Origin

IGP

EGP

Incomplete

2: AS_Path

AS_SET

AS_SEQUENCE

AS_CONFED_SET

AS_CONFED_SEQUENCE

3: Next_Hop

Next-hop IP address.

4: Multi_Exit_Disc

MED that is used to identify the optimal route for the traffic to enter an AS.

5: Local_Pref

Local_Pref that is used to identify the optimal route for the traffic to leave an AS.

6: Atomic_Aggregate

The BGP speaker selects the summary route rather than a specific route.

7: Aggregator

Router ID and AS number of the device that performs route summarization.

8: Community

Community attribute.

9: Originator_ID

Router ID of the originator of the reflected route.

10: Cluster_List

List of the RRs through which the reflected route passes.

14: MP_REACH_NLRI

Multiprotocol reachable NLRI.

15: MP_UNREACH_NLRI

Multiprotocol unreachable NLRI.

16: Extended Communities

Extended community attribute.

Error code

1 octet

Indicates an error type. The value 0 indicates a non-specific error type. For details about the error codes, see Table 8.

Error subcode

1 octet

Provides further information about the nature of a reported error.

Data

Variable

Indicates the error data.

1: message header error

1: Connection is not synchronized.

2: error message length.

3: error message type.

2: open message error

1: unsupported version number.

2: incorrect peer AS.

3: incorrect BGP identifier.

4: unsupported optional parameter.

5: authentication failure.

6: unacceptable hold time.

7: unsupported capability.

3: update message error

1: malformed attribute list.

2: unrecognized well-known attribute.

3: missing well-known attribute.

4: attribute flag error.

5: incorrect attribute length.

6: invalid origin attribute.

7: AS routing loop.

8: invalid Next_Hop attribute.

9: incorrect optional attribute.

10: invalid network field.

11: malformed AS_Path.

4: hold timer expired

0: successive messages not received within the period specified in the Hold Time field.

5: FSM error

0: successive messages not received within the period specified in the Hold Time field.

6: cease

1: The number of prefixes exceeded the maximum.

2: administrative shutdown.

3: peer de-configured.

4: administrative reset.

5: connection rejected.

6: other configuration change.

7: connection collision.

8: resource run-out.

9: BFD session down.

Marker

16 octets

Indicates whether the information synchronized between BGP peers is complete. This field is used for calculation in BGP authentication. If no authentication is used, the field is set to all ones in binary format or all Fs in hexadecimal notation.

Length

2 octets

Indicates the total length of a BGP message (including the header), in octets. The length ranges from 19 octets to 4096 octets.

Type

1 octet

Indicates the message type. The value of this field is an integer ranging from 1 to 5. The value of the Type field in each Keepalive message is 4.

AFI

2 octets (unsigned integer)

Indicates the address family ID, which is defined the same as that in Open messages.

Res.

1 octet (unsigned integer)

Must be all zeros. The field is ignored when a Route-refresh message is received.

SAFI

1 octet (unsigned integer)

Is defined the same as that in Open messages.